In order to fully test the radio equipment of a cellular telephone system, some type of radio test equipment is employed. In the testing of a base station of an analog cellular radio telephone system, a radio test loop is normally established between a base station transmitter and a corresponding base station receiver that are ordinarily used for traffic. Since both the transmitter and the corresponding receiver are designed to operate on the same RF duplex channel, the transmitter and receiver can be tested against each other. Such an arrangement reduces the cost of the test equipment, because no extra receiver or transmitter is needed (the RF frequency just has to be transposed by using a mixer) to test the base station transmitter and its corresponding receiver.
In a digital mobile system such as the GSM type employed in Europe or the TDMA systems being introduced in the United States, the base station transmitter transmits a RF carrier signal having a plurality of downlink messages assigned to a plurality of different time slots or "channels", while the base station receiver receives a different RF carrier signal having a plurality of uplink messages assigned to different "channels". Because the time slots of the transmitter and receiver are offset from each other in time, a time slot of the transmitter can not be directly linked to the same time slot of a receiver. Because the base station transmitter and receiver are operating at different RF carrier frequencies and on different channels, a radio test loop cannot be easily established between the transmitter and receiver of a base station.
In some TDMA systems, the problem is even more difficult, because the channels may use a multitude of RF carriers, i.e., the channels are so-called frequency hopping channels. If an ordinary traffic transmitter in a frequency "hopping" system is used as part of a radio test loop, such a radio test loop would cause interference with all "channels" hopping to the RF carrier signal being tested.
Accordingly, there is a need for a method of testing a base station using a low cost radio frequency test loop that does not require any additional expensive signal processing equipment.